Frequency modulator



Dec. 25, 1951 B. E. LENEHAN FREQUENCY MODULATOR Filed Dec. 13, 1946 INVENTOR Bernard 5 Leneban.

ATTORNEY Patented Dec. 25, 1951 FREQUENCY MODULATOR Bernard E. Lenehan, Bloomfield, N. J., asslgnor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application December 13, 1946, Serial No. 716,045

13 Claims.

This invention relates to electric discharge apparatus, more particularly to apparatus for producing frequency-modulated oscillations, and it has for an object to provide improved apparatus of this kind.

Apparatus constructed in accordance with the teachings of the prior art, of which I am aware, is exemplified 'by the Fichandler Patent 2,000,584. Fichandler discloses a system for producing frequency-modulated oscillations comprising an oscillatory circuit or frequency-determining network, across which a variable resistance and a reactance are connected. The components of the network and the resistance and reactance are so set that variation of the resistance over a small range produces variation of the reactance of the whole network without accompanying variation of resistance. Fichandler shows the variable resistance as a three-electrode tube, the grid potential of which is varied to vary the plate resistance.

A transmitter including the Fichandler system has several undesirable features. The output of the transmitter is distorted. It is marred by hum such as arises from undesired pulsations of a 60-cycle power supply and by other sounds not introduced by the modulating source. Amplitude modulations are also at times present in the output.

-It is accordingly an object to provide a frequency-modulation system which shall produce signals substantially free of distortion.

Another object is to provide, a frequencymodulation system which shall produce signals substantially free of attending current hum.

A further object is to provide a frequencymodulation system in which extraneous sounds are substantially suppressed.

A still further object is to provide a frequencymodulation system which shall produce signals substantially free of amplitude modulation.

-It is realized that the sensitive point in a frequency-modulation transmitter is the network at which the modulations are introduced. Beyond this point the frequency multiplication is high. Distortion arising from interelectrode capacity at the modulation network, alternat ing-current hum, microphonic noises and amplitude modulations introduced at the network are highly amplified by the multiplication effect and materially deteriorate the output.

The interelectrode capacity of Fichandlers resistance tube is substantial, as is also the capacity to ground of the electrodes. These capacities introduce distortion into the modulations impressed on the frequency-determining network. The amplitude of the distortions is small, but the distortions are highly amplified by the frequency multiplication and deteriorate the output of the transmitter.

Fichandlers resistance tube requires a filament sup-ply which is customarily ofthe rectified alternating-current type. Although this supply is filtered, the filament current retains pulsations of low -magnitude. The pulsations, highly amplified, introduce-uh esirable hum in the output. l

The resistance tube has electrodes which have a tendency to vibrate. The sounds which are produced in the ordinary transmitting station and, in particular, in the vicinity of an ordinary communication equipment (aboard a plane or ship or in a police car) tend to cause the electrodes to execute vibrations of minute amplitude. The output of the modulator is highly amplified, and the minute vibrations are converted into substantial microphonic sounds.

I'n-Fichandlers system energy from the oscillation generator flows back with facility to the modulating network. The variable resistance tube (34, Fig. 5), having substantial anode-grid capacity, provides a relatively low impedance feedback path. The regeneration effect produced by the feedback amplified by mul tiplication also deteriorates the output.

The prior art is further exemplified by the Idzerda Patent 1,648,402. Idzerda makes use of the principle that resistance variation in highfrequency oscillatory circuits causes frequency variation. To obtain this, a microphone is arranged in an oscillator circuit in such manner that the resistance variations caused by the sound waves in the microphone give rise to irequency-variations. The arrangement disclosed in this patent is satisfactory, that is, with the microphone connected directly across the irequency-determining network, as shown; however, ifit is desired to modulate the circuit from a point remote from the oscillatory circuit, this arrangement fails to produce modulation because the resistance variations of the microphone are lost in the capacitance of the wires connecting the microphone to the oscillatory circuit.

In accordance with my invention, I provide a frequency-modulating system for a frequency range such as used in carrier-current communications wherein this objection is eliminated by providing a fixed resistance and a rectifier having a non-linear impedance characteristic in determining or oscillatin circuit in is in parallel with the inductance I9 insteadof in parallel with the condenser [6 as in the previous embodiment. In this embodiment, the condenser I6 is made variable for adjusting the "circuit In for resonance. In all other respects this embodiment is identical in construction and operation tothe previous embodiment. i Fig. 3 illustrates an embodiment of my invention using a single rectifier unit 3|. In all other respects this embodiment is identical in construction and operation to the embodiment illustrated in Fig. 1. a i

-' In Figs. 2 and 3 arrows indicate connections to the anode l2, cathode l3, and grid 14 of the discharge device I I. I

I From the foregoing description it is seen that I have provided a simple and efiicient method of frequency-modulating an oscillatory circuit from a, remote point to produce signals substantially free of distortion, alternating-current hum, extraneous noises, and amplitude noises.

While I have shown my invention in several forms, it will be obvious to those skilled in the art that it is not so limited but is susceptible of various other changes and modifications without departing from the spirit thereof, and I desire, therefore, that only such limitations shall be placed thereupon as are specifically set forth in the appended claims.

I claim as my invention:

1. In combination, a generator of electrical oscillation comprising a frequency-determining network, a source of audio frequency for modui lating said generator, means connecting between said audio source and said generator comprising a network shunting said frequency-determining network and including a full wave rectifier the resistance of which varies as the current there through varies, and means remotely of said generator for causing the current flowing through said rectifier to vary.

2. In combination, a generator of electrical oscillations including a frequency-determining network, means for effecting frequency modulation of said oscillations comprising a full wave rectifier connected in shunt with said frequencydetermining network, a source of modulating frequency connected to said rectifier, and a resistance connected in series with said modulating source and said rectifier.

3. In combination, a generator of electrical oscillations including a frequency-determining network, means for effecting frequency modulation of said oscillations comprising a barrierlayer rectifier connected in shunt with said frequency-determining network, a source of modulating frequency connected to said rectifier, and a resistance connected in series with said modulating source and said rectifier.

4. In combination, a generator of electrical oscillations including a frequency-determining network comprising a four terminal full wave rectifier having two of its terminals connected in shunt with said frequency-determining network, a source of modulating frequency connected to the other two terminals of said rectifier, and a resistance in series with said modulating source.

5. In apparatus including a frequency determining network for providing a frequency modulated output voltage, the combination of a first voltage source, said first voltage being proportional to said output voltage, a variable imped ance including a rectifier network having an inputc'ircuit and an output circuit, a fixed ind-- pedance, said input circuit being connected in shunt across said first voltage sourceand said output circuit being connected to apply a first direct current voltage across said fixed impedance, and a modulating voltage source connected in series with said output circuit and said fixed impedance to apply a second direct current voltage across said fixed impedance. i 6. The combination as claimed in claim 5, in which the rectifier network comprises a full wave rectifier. 7 7. In apparatus for providing a frequency modulated output voltage, the combination of a source of alternating current voltage which is proportional to said output voltage, a resistance member, a full wave rectifier network connected between said alternating current voltage source-and said resistance memberand adapted for applying a first direct current voltage across said resistance member, a source of a modulating voltage, and connections for applying a second direct current voltage across said resistance member proportional to said modulating voltage.

8. In apparatus for providing a frequency modulated output voltage, the combination of an alternating current voltage source, said alternating current voltage being proportional to said output voltage, a variable impedance comprising a full wave rectifier network having an input circuit and an output circuit, said input circuit being connected across said alternating current voltage source, a fixed impedance connected to said output circuit such that a first direct current voltage is applied across said fixed impedance, a modulating voltage source, and connections between said modulating voltage source and said fixed impedance such that a second direct current voltage is applied across said fixed impedance.

9. The combination as claimed in claim 8, with the second direct current voltage being applied across the fixed impedance in a direction such that a resultant direct current voltage occurs which has a magnitude that determines the effective frequency modulation of said output voltage.

10. In apparatus including a frequency determining network for providing a frequency modulated output voltage, the combination of a first voltage source, said first voltage being proportional to said output voltage from said frequency determining network, a variable impedance comprising a rectifier network having an input circuit and an output circuit, said input circuit being connected in shunt across said first voltage source, a second impedance connected to said output circuit such that a first current is established in said second impedance, and a modulating voltage source connected in series with said second impedance and said output circuit to establish a second current in said second impedance.

11. In apparatus including a frequency determining network comprising a tuned capacitance and inductance circuit for providing a frequency modulated output voltage, the combination of a first voltage source comprising a portion of said frequency determining network, a variable impedance comprising a rectifier having an input and an output, said input being connected in shunt across said first voltage source, a second impedance connected to said rectifier output such that a first current level is established in said second impedance which is proportional to said output voltage, and a source of modulating voltage connected in series to said second impedance and saidreetifler out ut such, that: arsecond zcurf' level :isvthereby established 3in. :swid: :seQQnQ;

iz'mnedame;v to determine themod-nletion of: Sai outnutivoltage.

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1:3 apparatus :for, providing a frequency modulated output. voltage, the combination, :ofga. sQurceof-e firstjvoltege whiohgisroportional te sa idqqutput voltage, an impedancemember, .afull.

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BERNARD-. E. :LENEHAN.

8 flummucusuclmn renewi g references of r'ecera n -me flle of this patent; V e

' "um'rrnn' STATESPATENTS FOREIGN PATENTS V Country" Dale 561,322 GreatfBIiim'in .15, 194.4 

